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Liver Tumors and Cancers in Dogs and Cats
Revised: October 18, 2023
Published: July 31, 2009

Confirming a liver tumor requires a medical diagnostic journey starting with a sick pet or perhaps an abnormal lab test.  From these beginnings, the journey proceeds through further blood tests and, ultimately, imaging. Through imaging of the abdomen (stomach), the liver lobes are evaluated, and a tumor can be verified. Through imaging of the abdomen, the liver lobes can be evaluated and a tumor verified. We will assume that you are reading this because the diagnostic process has largely been completed, and a tumor has been found. At this point, undoubtedly, many questions have emerged and must be answered in order to make proper choices.

  • Is this a primary tumor (meaning it arose in the liver) or is it the result of cancer spread from a primary tumor elsewhere?
  • Is the tumor benign or malignant?
  • Is the tumor of a size where surgery is a reasonable option for palliation, if not cure?
  • How is the tumor affecting the day-to-day life of the pet, and how will this change in the future?
  • How long will the pet be expected to live with or without treatment?

Ultrasound Can Answer Many Questions

Many of these questions can be answered by ultrasound, the most common medium for evaluating the texture of the liver. Unlike radiography, where all soft tissue appears as the same texture, ultrasound is able to separate out tissues depending on their water content. This means that it is possible to see inside the liver and see if there is one tumor or many and how much normal liver texture is left. Ultrasound can determine what organs show tumors inside, whether or not local lymph nodes are enlarged (which could indicate spread of the tumor), and whether or not surgery would be useful. It is also possible to take either a needle aspirate or an actual biopsy to determine the type of tumor, and knowing the type of tumor will answer remaining questions regarding treatment options, survival time, etc. In other words, if an ultrasound of the liver has not been done yet, it is time to do it.

What Happens During an Ultrasound Scan

Sedation is not usually required for imaging or even tissue sampling. The fur is shaved for best contact with the ultrasound probe. Watch Smudge, the cat, have a liver scan in this short video.

What is the Difference between a Needle Aspirate and a Biopsy?

A needle aspirate involves sticking a long needle into the abnormal area and withdrawing cells for analysis. This is usually done with ultrasound guidance so as to avoid hitting any large blood vessels and to make sure the desired area is sampled. The advantage of the needle aspirate is that anesthesia or sedation of the sick patient is generally not necessary, there is less potential for bleeding than with biopsy, and results may be obtained as soon as overnight in many cases. The disadvantage is that the sample obtained consists of cells only, and the architecture connecting these cells is lost. Diagnosis will be less specific and may consist of conceptual information like benign vs. malignant, inflammatory vs. not-inflammatory, etc. Some tumors, such as the mast cell tumor or lymphoma, readily release their cells, and architecture is not needed for diagnosis. Other tumors are not so readily distinguished, and there is greater potential for a frustrating non-diagnostic sample result than there is with a biopsy.

A biopsy yields a very different sample: a chunk of tissue. In this sample, the microscopic structures of the liver can be viewed, as can the cells infiltrating them. The architecture of the tissue is preserved. Tumors can be graded for the degree of malignancy, plus the specific type of tumor is revealed. The downside stems from the fact that a larger piece of tissue is required. Clotting tests must be run prior to the procedure to ensure that the liver will not bleed. A different type of needle is used, and some sort of sedation is typically needed, which may add risk. Results typically take longer for a biopsy sample as the laboratory preparation is more complicated. Expense is generally greater for biopsy than for aspirate.

The ultrasonographer is typically in a good position to determine the best balance of risk, expense, and potential information yield.

A correct diagnosis is obtained in 60% of needle aspirates of the liver. A correct diagnosis is obtained in 90% of liver biopsies. Bleeding occurs in 5 percent of sampled patients, with bleeding graded as moderate in those that had biopsies and mild in those that had aspirates.

Metastatic Cancer

If cancer is found in the liver but is believed not to have originated there, this indicates cancer spread and advanced disease. Prognosis is poor, though what options remain depend on the type of cancer. Metastatic disease in the liver is approximately two and a half times more common than primary cancer in the liver, with most tumors having spread from the spleen, pancreas, or intestinal tract.

Liver cancer is an area that not all veterinarians are comfortable treating. Discuss with your veterinarian whether a referral to an oncologist would be best for you and your pet.

Classifying Primary Tumors of the Liver

Again, a primary liver tumor is a tumor that arose in the liver (rather than having spread there from a primary tumor elsewhere). Primary tumors are classified by their shape/configuration within the liver and by the type of liver tissue they originated from. Tumors may be massive, nodular, or diffuse. The best one to have is the massive type, as it is in one area and is thus the most amenable to surgical removal. A diffuse tumor involves the entire liver evenly, while a nodular tumor forms discreet bumps within the liver. While ultrasound can tell us if a tumor is massive, nodular, or diffuse, it cannot tell us the tissue of origin.

The four tissues of origin for primary liver tumors are hepatocellular, bile duct, neuroendocrine (also called carcinoid), and mesenchymal.

Hepatocellular Tumors (the Most Common Liver Tumors in Dogs and Cats)

There are three types of hepatocellular tumors that dogs and cats can have: hepatocellular carcinoma (the most common hepatocellular tumor of dogs), hepatocellular adenoma (the most common hepatocellular tumor of cats), and the hepatoblastoma (which is exceedingly rare and has only been reported twice).

The hepatocellular adenoma is benign and does not cause illness. It might cause some blood changes that in turn can trigger a medical work up, but if a biopsy turns up this tumor in an otherwise healthy pet, the news is good and nothing bad should be expected to come of it. Alternatively, if this diagnosis is made in a pet that is sick, the illness probably cannot be blamed on this tumor and a further search for the right diagnosis is warranted.

The hepatocellular carcinoma is an important tumor of dogs and cats, not only because it is moderately common but because it tends to be amenable to surgery even though it is malignant. In humans, this tumor often has a viral basis (i.e. one of the hepatitis viruses) as well as an association with cirrhosis (scarring in the liver) but in dogs and cats neither of these associations holds true.

Most hepatocellular carcinomas (50-80%) are massive, 16-25% are nodular, and 19% are diffuse. In dogs, the rate of metastasis (cancer spread) is 90-100% for cases with either nodular or diffuse hepatocellular carcinoma but only 37% for those with massive disease.

Phorto of a veternary surgeon operating on a dog
Dr. Brooks removes hepatocellular carcinoma. Tumor size is characteristically large. Photo courtesy of MarVistaVet

If the dog or cat has a massive tumor, surgery can greatly improve life quality even if the entire tumor cannot be removed. The hepatocellular carcinoma grows slowly, so surgery, while challenging, typically produces excellent results. The risks of surgery include bleeding, circulatory compromise to the remaining liver portions, reduced liver function after surgery, and transient low blood sugar. The time of greatest risk and concern is the time of the surgery and during surgical recovery. After recovery, one study found a median survival time of 1460 days (4 years). If the tumor is on the right side of the liver, surgery is more difficult because the vena cava, the largest vein in the body, is close by and may bleed. The bottom line is that massive disease should be addressed with surgery with the potential for cure, while nodular or diffuse disease has a poor prognosis.

Bile Duct Tumors

There are two types of bile duct tumors to be had by dogs and cats: biliary adenoma (benign) and biliary carcinoma (malignant.) The biliary adenoma is the most common primary liver tumor in cats and accounts for over 50 percent of all feline primary liver tumors. They are cystic in structure, meaning they tend to be large and fluid-filled. They do not cause problems until they are so big that they press on other organs, but because they are fluid-filled, the fluid can be periodically sucked out with ultrasound guidance to restore health, or the tumor can be removed surgically once and for all.

The biliary carcinoma can be massive, nodular, or diffuse. If the tumor is massive, surgery is generally recommended, though this tumor is so malignant that survival times after surgery are typically only about 6 months. Metastasis ultimately occurs in just under 80 percent in cats and just over 80 percent in dogs.

Neuroendocrine Tumors

These tumors are rare and usually diffuse (2/3 of cases are diffuse and 1/3 are nodular). They tend to spread quickly and early in their course. The prognosis is poor, and because they are rare tumors, few studies are available to suggest treatment.

Mesenchymal Tumors

The word mesenchyma (mezz-en-KY-ma) does not exactly roll off one's tongue. The mesenchyma of the liver is the general liver tissue separate from the bile ducts and some other parts inside the liver. Tumors of the mesenchyma are called sarcomas.

Primary sarcomas of the liver are unusual but include hemangiosarcoma (a malignancy of blood vessels, which is a common secondary tumor, but only 5 percent of hemangiosarcomas arise in the liver), fibrosarcoma (malignancy of fibrous tissue), osteosarcoma (bone malignancy), and leiomyosarcoma (smooth muscle malignancy).

Approximately 36 percent of sarcomas are massive, and 64 percent are nodular, with metastasis found in 80-100 percent of cases, depending on the study. Massive tumors of other types are generally amenable to surgery, but in the case of sarcomas, the tumor has already begun to spread. Some tumors can be treated with chemotherapy.

One Last Tumor

The myelolipoma of cats is worth mentioning as it carries an excellent prognosis with surgery. This is a benign and often large tumor consisting of fat mixed with blood cell precursors. Current theory is that it develops in response to poor oxygenation in a particular area. Once removed, it should not grow back.

Hopefully, this summary has been helpful in reviewing the possible tumors and what to expect based on how they appear on ultrasound.

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